The control of particulate emissions from diesel engines is one of the major technical issues facing the automotive industry if diesel engines are to be used in light duty vehicles. Some form of exhaust treatment will be necessary to meet the above regulations. Such exhaust treatment is typically envisioned to consist of a filter or trap to collect the particulates in the exhaust, together with a method for periodically disposing of the collected particulates. The system is commonly called a trap oxidizer. Periodic disposal of particulates is necessary because as the particulates collect on the trap the exhaust back pressure increases and adversely affects fuel economy and vehicle performance. Particulates can collect in amounts of up to about 75 gallons over a period of 50,000 miles of vehicle operation for a conventional vehicle diesel engine.
To initiate particulate oxidation the exhaust gas temperature must be at a very elevated level. Unfortunately in typical diesel powered passenger cars the exhaust gas temperature usually does not attain a high level until speeds above 70 mph are reached. Therefore supplementary means has appeared necessary to the prior art to achieve oxidation of such particulates.
Attempts by the prior art to provide for particulate oxidation have included the idea of using a separately fueled burner stationed in the particulate trap and which is selectively controlled to raise the inlet temperature of the trap to a desired temperature for regeneration (the latter is a term that is used to identify the oxidation and removal of the particles).
One technique is to initiate trap regeneration by using the exhaust gas from the diesel engine to produce the required temperature and oxygen concentration. One example of this is shown in U.S. Pat. No. 3,800,772, which shuts off fuel to certain combustion cylinders of the diesel engine, allowing the inducted air to continue through such fuel starved cylinders to become part of the exhaust gas. In U.S. Pat. No. 4,211,075, fuel as well as air is stopped from entering certain selected combustion chambers with the hope that the remaining combustion chambers will cause an increase in temperature in the exhaust gas sufficient to provide for regeneration. However, diesel engines, particularly those of the indirect injection type, have high compression/expansion ratios of over 20:1 and operate unthrottled with lean air/fuel ratios. Thus shutting off certain of the cylinders with fuel or air, or both, is inadequate to initiate the proper degree of regeneration at most speed and loading conditions encountered in normal driving. Certain operating conditions can achieve the desired regeneration, but these conditions are usually achieved only under special circumstances. Thus high speed/high load regeneration is not practical for normal vehicle operation and certainly not for steady state conditions. Furthermore, throttling generally has an adverse effect on the engine exhaust emissions and fuel consumption.
To solve the problems associated with throttling, the prior art has stationed a burner at the upstream portion of the trap, which is separately fueled for creating the proper temperature environment for regeneration. The burner can be supplied with air from an external pump or the total amount of exhaust gas from the entire engine can be used as the supplier of the excess oxygen needed for combustion in the particulate trap. In either case the externally fueled burner is supplied with combustible fuel which is sprayed through an atomizing nozzle and then ignited by a glow plug or spark plug.
A disadvantage to utilizing an air fed burner regeneration system is that considerable complexity is added to the engine to provide for an engine driven air pump. In addition, the stability of the regeneration operation is in question for engine speeds above 55 mph. An exhaust supplied burner regeneration system produces somewhat higher hydrocarbon emissions. With each of these systems the consumption of additional fuel beyond that required for normal engine operation is excessive, requiring 2.5 to 5% of the fuel required for normal engine operation over the federal CVS driving cycle.